This invention relates to an infrared interference type film thickness measuring method and instrument for measuring the thickness of a thin polymer film by utilizing an interference fringe of an infrared light caused by the film.
For continuous, on-line non-contacting measurement of the thickness of a polymer film on the film manufacturing production line, the following two methods have widely been employed:
(1) A method of measuring the thickness of the polymer film utilizing absorption and scattering of beta rays by the film;
(2) A method of measuring the thickness of the polymer film utilizing absorption of infrared rays by the film.
However, the abovesaid two methods have the following defects:
The method utilizing beta rays has the defect that as the thickness of the polymer film to be measured decreases, that is, when the film thickness is less than about 10 to 30.mu., an appreciable error is produced between the measured thickness and the actual one. That is, this method is to make use of absorption and scattering of beta rays corresponding to the weight of the member to be measured, so that the film thickness is measured by dividing the weight of the film per unit area by the density of the film. Therefore, no problems occur when the member to be measured is a metal or the like which has a high density; but in the case of a polymer film whose density is close to 1, its weight is very small, and the rate of beta rays being absorbed and scattered by the film becomes very low, so that the measurement is greatly affected by the absorption and scattering of the beta rays by air and variations in the intensity of the beta ray source which stem from a statistical decay of beta rays. As a consequence, in the case of using beta rays for the film thickness measurement, variations in the scattering and absorption of beta rays resulting from changes in the thickness of the polymer film are measured with low SN ratio (signal-to-noise ratio); in particular, measurement of the thickness of a thin polymer film is difficult.
The method utilizing infrared radiation is safe and superior, as a method of measuring the thickness of a polymer film, to the method using beta rays because it employs infrared rays unsusceptible to the influence of air in place of radiation such as beta rays. This method adopts the principle of measuring the thickness of a film utilizing absorption of chromatic infrared light by the film corresponding to its thickness. Consequently, in the case of measuring the thickness of a thin polymer film, when the film comes to satisfy the condition of interference of a parallel-surfaced thin film, the infrared light cast thereon undergoes multiple reflection in the film to cause a light interference phenomenon, and when the film thickness varies, the interference condition changes, and the infrared light transmitted through the film becomes light when light rays are in-phase, and dark when the rays are out-of-phase. This variation in the intensity of the infrared light is utterly undistinguishable from the variation in the absorption of the infrared light by the polymer film corresponding to the film thickness, resulting in a large error in the measured film thickness. Accordingly, with the method utilizing the chromatic infrared light corresponding to the wavelength at the peak of absorption by the polymer film, it is difficult to measure the thickness of a polymer film, which causes marked light interference, whose thickness is very small and whose surfaces are highly parallel and flat and lustrous and which has a high degree of transparency.
Accordingly, in connection with the method for film thickness measurements utilizing the infrared absorption, there have heretofore been made various inventions for reducing the interference phenomenon by infrared radiation in a thin film which causes an error in the measurement, as seen from U.S. Pat. Nos. 3,631,526, 3,693,025, 3,973,122 and 4,027,161.
There has been known, in general, a method of measuring the film thickness by making use of the light interference phenomenon by a film. For example, U.S. Pat. No. 3,238,839 discloses a method of measuring the thickness of a thermoplastic film utilizing the interference. In this patent, as described in its patent gazette, col. 4, lines 14-16, the wavelengths of light used for measurement are between 0.435 and 0.546.mu., which are visible rays of light. In the case of using such visible light, the measurement is liable to be affected by external light and becomes less reliable and requires a complicated measuring instrument. Further, as the wavelength used becomes shorter, the amplitude and period of the resulting interference waveform decrease; namely, the interference waveform becomes hard to obtain, so that the measurement becomes difficult by that. Moreover, in this patent the interference waveform is displayed, and the operator counts the wave number N from the display, and the wave number N is used for calculation to obtain the film thickness. That is, the measuring instrument of this patent does not provide directly the film thickness as the measured result but requires counting of the wave number N by the operator. Accordingly, such a method is not suitable for on-line measurement of the thickness of a film moving on the manufacturing production line. On top of that, the measuring instrument is fixed, so that it is impossible to measure the thickness of the moving film over the entire width thereof.
U.S. Pat. No. 3,601,492 also discloses apparatus for measuring film thickness utilizing the interference phenomenon. This apparatus is to measure the thickness of a silicon wafer by making use of interference of an infrared light, and the interference fringe obtained is displayed on an oscilloscope and marker lines are set visually at the displayed interference fringe. Since the marker lines are set by the eye, the measured value varies for each setting and according to the particular operator, resulting in inaccurate measurement; furthermore, since manual operation is needed, the measurement is time-consuming and hence low in measuring speed. Moreover, the member to be measured and the measuring instrument are both fixed, so that it is impossible to measure the thickness of a moving member while moving the measuring instrument relative thereto. The infrared wavelength used in this patent is in the range of 11 to 35 .mu.m. This wavelength range is one that if it is used for measuring the thickness of a polymer film, there exist a number of strong characteristic absorbing peaks according to the particular polymer film; therefore, there is a fear that the measurement cannot be achieved.
An object of this invention is to provide an infrared interference type film thickness measuring method and instrument with which it is possible to measure, with high accuracy, the thickness of a thin polymer film such that when it is measured by the method utilizing beta rays, a large error is resulted in the measurement and that also when it is measured by the method utilizing infrared absorption, a large error is produced in the measurement due to the interference phenomenon.
Another object of this invention is to provide an infrared interference type film thickness measuring method and instrument which permits measurement of the film thickness with substantially no manual operation.
Still another object of this invention is to provide an infrared interference type film thickness measuring method and instrument with which it is possible to continuously measure, with high accuracy the thickness of a moving polymer film in a non-contacting manner.